1. Field of the Invention
The present invention generally relates to photovoltaic cells and, more particularly, to an improved photovoltaic cell having an active silicon or silicon-germanium substrate subcell that results in improved adaptability, yield and efficiency.
2. Description of Related Art
The interest in photovoltaic (PV) cells continues as concerns over pollution and limited resources continue. The continued interest has been in both terrestrial and non-terrestrial applications. In the non-terrestrial environment of outer space, the concern over limited resources of any type is a major one. This is because the need to increase the amount of a resource increases the payload. An increased payload can increase the cost of a launch more than linearly. With the ready availability of solar energy in outer space for a spacecraft such as a satellite, the conversion of solar energy into electrical energy is an obvious alternative to increased payload.
The cost per watt of electrical power generation capacity of photovoltaic systems is the main factor which inhibits their widespread use in terrestrial applications. Conversion efficiency of sunlight to electricity is of critical importance for terrestrial photovoltaic systems, since increased efficiency means that all area-related components of the electricity generation system, i.e., cell area, module or collector area, support structures, land area, etc., can be reduced when the efficiency is increased, for a given required power output of the system. For example, in concentrator photovoltaic systems which concentrate the sunlight, typically from 2 to 2000 times, onto the photovoltaic cell, an increase in efficiency means that the area of the expensive concentrating optics can be reduced proportionally. Concentrator photovoltaic systems are a likely application of the multijunction cells described in this invention, because the cell area can be reduced by the concentration ratio, allowing the use of relatively complicated solar cells with a high cost per unit area.
Irrespective of the application, and as with any energy generation system, efforts have been ongoing to increase the output and/or efficiency of PV cells. In terms of output, multiple cells or layers having different energy bandgaps have been stacked so that each cell or layer can absorb a different part of the wide energy distribution of photons in sunlight. The stacked arrangement has been provided in a monolithic structure on a single substrate or on multiple substrates. Examples of multi-cell devices are shown in Kurtz et al., xe2x80x9cModeling of two-junction, series-connected tandem solar cells using top-cell thickness as an adjustable parameter,xe2x80x9d J. Appl. Phys. 68(4), pp. 1890-1895, Aug. 15, 1990; and U. S. Pat. Nos. 5,800,630; 5,407,491; 5,100,478; 4,332,974; 4,225,211; and 4,017,332.
In the multiple cell device, semiconductive materials are typically lattice-matched to form multiple p-n (or n-p) junctions. The p-n (or n-p) junctions can be of the homojunction or heterojunction type. When solar energy is received at a junction, minority carriers (i.e., electrons and holes) are generated in the conduction and valence bands of the semiconductor materials adjacent the junction. A voltage is thereby created across the junction and a current can be utilized therefrom. As the solar energy passes to the next junction which has been optimized to a lower energy range, additional solar energy at this lower energy range can be converted into a useful current. With a greater number of junctions, there can be greater conversion efficiency and increased output voltage.
For the multiple-cell PV device, efficiency is limited by the requirement of low resistance interfaces between the individual cells to enable the generated current to flow from one cell to the next. Accordingly, in a monolithic structure, tunnel junctions have been used to minimize the blockage of current flow. In a multiple wafer structure, front and back metallization grids with low coverage fraction and transparent conductors have been used for low resistance connectivity.
Another limitation to the multiple cell PV device is that current output at each junction must be the same for optimum efficiency in the series-connected configuration. Also, there is a practical limit on the number of junctions, since each successive junction generates a smaller current.
Whether in the multiple-junction or single-junction PV device, a conventional characteristic of PV cells has been the use of a single window layer on an emitter layer disposed on a base/substrate, which is shown for example in U.S. Pat. No. 5,322,573. Similarly, a single layer back-surface field structure below the base/substrate has been used, as shown in U.S. Pat. No. 5,800,630. The purpose of the back-surface field structure has been to serve as a passivation layer, like the single window layer described above.
The concern over efficiency in PV cells has created more interest in the use of germanium, gallium arsenide, indium phosphide, and gallium indium phosphide, all of which have been thought to be more efficient than silicon. Indium phosphide has another perceived advantage of being radiation resistant, which is of particular benefit in space applications. However, silicon is stronger, less expensive, and less than half as dense as Ge and GaAs substrates. Accordingly, silicon remains highly viable for continued use and is discussed, for example, in Hayashi et al., xe2x80x9cMOCVD Growth of GaAsP on Si For Tandem Solar Cell Application,xe2x80x9d First WCPEC, pp. 1890-1893 (1994) and Wojtczuk et al., xe2x80x9cDevelopment of InP Solar Cells on Inexpensive Si Wafers,xe2x80x9d First WCPEC, pp. 1705-1708 (1994).
As can be seen, there is a need for an improved multifunction photovoltaic cell that is thinner and lighter than conventional solar cells, which has increased efficiency, yield, and adaptability to different applications.
The present invention is generally directed to an improved multijunction photovoltaic cell in which a pure silicon or silicon-germanium or pure germanium substrate serves as an active subcell. The active substrate can be located at the top, bottom, or an intermediate position within the cell. The active substrate has the ability to provide photogenerated current density and voltage in addition to structural support. The individual subcells that make up the multijunction solar cell may be of the heterojunction or homojunction types, and may have either an n-on-p or a p-on-n configuration.
In a series-interconnected multijunction photovoltaic cell, a design consideration of critical importance is for each subcell to have roughly the same photogenerated current density, so that the cell with the lowest photogenerated current density does not limit the current flowing through the other subcells in the multijunction cell. Accordingly, an important component of this invention is the selection of the number of subcells above and below the substrate subcell, and selection of the combination of bandgaps of each subcell determined by the choice of subcell composition and lattice constant, in order to achieve current matching with the substrate which is preferably composed of silicon (Si) or silicon-germanium (SiGe) or germanium (Ge). More preferably, the substrate is composed of Si or SiGe.
In the present invention, the active silicon or silicon-germanium or germanium substrate typically has one side which is more responsive to incident light than the opposite side, and this side is referred to as the xe2x80x9cactive sidexe2x80x9d in the description that follows. Note, however, that the opposite side can also have some degree of photoresponsivity to light incident on it. The active side of the substrate is typically the side closest to the voltage-producing p-n junction in the substrate.
Specifically, in one embodiment of the present invention, the improved photovoltaic cell includes an active silicon (Si) or silicon-germanium (SiGe) or germanium (Ge) substrate having one active side and characterized by a substrate bandgap; one or more subcells are disposed adjacent either the active side or the opposite side and current matched with the substrate, a transition layer intermediate the active side and the side of the subcell closest to it; and a transition layer between adjacent subcells if there is more than one subcell.
In a another embodiment, the photovoltaic cell of the present invention includes an active Si or SiGe or Ge substrate having an active upper side and characterized by a substrate bandgap; one or more lower subcells disposed adjacent the lower side and current matched with the substrate, with the lower subcell(s) typically having lower bandgap(s) than the substrate bandgap; and a transition layer intermediate the lower side and the lower subcell(s).
In still another embodiment, the improved photovoltaic cell includes an active Si or SiGe or Ge substrate having an active upper side and characterized by a substrate bandgap; one or more upper subcells disposed adjacent the upper side and current matched with the substrate, with the upper subcell(s) typically having bandgap(s) greater than the substrate bandgap; one or more lower subcells disposed adjacent the lower side of the substrate and current matched with the substrate, with the lower subcell(s) typically having lower bandgap(s) than the substrate bandgap; a transition layer intermediate the upper side and the upper subcell(s); as well as a transition layer intermediate the lower side and the lower subcell(s).
In a further embodiment, the photovoltaic cell of the present invention includes an active Si, SiGe, or Ge substrate characterized by a substrate bandgap that is lower than or equal to that of a pure silicon substrate and a substrate lattice constant that is larger than or equal to that of pure silicon; one or more upper subcells disposed adjacent the upper side of the substrate and current matched with the substrate, with the upper subcell(s) typically having bandgap(s) greater than the substrate bandgap; and zero, one, or more lower subcells disposed adjacent the lower side of the substrate and current matched with the substrate, with the lower subcell(s) typically having lower bandgap(s) than the substrate bandgap. The semiconductor materials of the upper and lower subcells are selected to have approximately the same lattice constant as the Si or SiGe or Ge substrate. In this embodiment, therefore, transition layers are not required to change from the lattice constant of the substrate to the lattice constant of the semiconductor materials in the upper and lower subcells. The case with no upper subcells, and one or more lower subcells lattice-matched to the active Si or SiGe or Ge substrate, is also included in this embodiment.
In yet another embodiment, the photovoltaic cell of the present invention includes an active Si, SiGe, or Ge substrate characterized by a substrate bandgap that is lower than or equal to that of a pure silicon substrate and a substrate lattice constant that is larger than or equal to that of pure silicon. Two or more groups of upper subcells, with each group in general having a lattice constant different from the substrate and different from the other subcell groups, are positioned above the upper surface of the active substrate, with a transition layer between each of the subcell groups, and between the lowermost subcell group and the substrate. As in the previous embodiments, the transition layers serve to change the lattice constant from one region of the multijunction cell, i.e. the substrate or one of the subcell groups, to the lattice constant of the adjacent subcell group. The ability to choose lattice constant of each group of subcells allows greater flexibility in the choice of bandgap of the subcells, which in turn facilitates matching the photogenerated current density of each subcell in the multifunction cell for a given spectrum of incident light. This combination of two or more groups of upper subcells with different lattice constants and two or more lattice-constant-transitioning layers can also be combined with the lower subcell configurations described in the previous embodiments.
In an additional embodiment, the photovoltaic cell of the present invention again includes an active Si, SiGe, or Ge substrate characterized by a substrate bandgap that is lower than or equal to that of a pure silicon substrate and a substrate lattice constant that is larger than or equal to that of pure silicon. Two or more groups of lower subcells, with each group in general having a lattice constant different from the substrate and different from the other subcell groups, are positioned below the lower surface of the active subcell, with a transition layer between each of the subcell groups, and between the uppermost subcell group and the substrate. This combination of two or more groups of lower subcells with different lattice constants and two or more lattice-constant-transitioning layers can also be combined with the upper subcell configurations described in the previous embodiments.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description, and claims.